As an example of conventional arts, an art described in Japanese Laid-Open Patent Publication 1998-124308 is disclosed (refer to Patent Document 1).
Regarding a control method of a conventional servomotor controller in a case where a power supply voltage falls off, in a device for driving joints of a robot body by servomotors that are disposed adjacent to the respective joints, the conventional art is described, in which each position of the joints can be accurately detected based on signals from a pulse encoder, even when the power supply voltage temporarily fails or abnormally falls off, by outputting an abnormal signal when an output voltage of a DC power supply serving as a servomotor driving power source supplied from a commercial power supply drops down to lower than a predefined value, and by stopping the servomotor, and by holding the stop position of the servomotor at that time.
[Patent Document 1]
Japanese Laid-Open Patent Publication 1990-76697